Nucleoside analogs (I) are new. Nucleoside analogs of formula (I) and their salts are new. [Image] A : bond, 1-4C alkylene, O(CH 2) m or NR 4(CH 2) n； m, n : 1-3； B : aromatic heterocyclyl or carbocyclyl both optionally substituted； R 1-R 4H, alkyl, alkenyl, cycloalkyl, aryl, aralkyl, acyl, silyl, optionally protected phosphoric acid or PR 7R 8； or R 1-R 3amino protecting group； or R 3hydroxy protecting group； and R 7, R 8amino (optionally substituted by 1-5C alkyl), 1-5C alkoxy, 1-5C alkylthio, 1-6C cyanoalkoxy or optionally protected SH or OH. The protecting groups are for use in the synthesis of nucleic acids Independent claims are also included for: (1) nucleoside analogs of formula (II) and their salts； and (2) oligonucleotides and their salts comprising at least one nucleoside residue of formula (IA) or (IIA). [Image] R 5, R 6a group R 1 and R 2 respectively.
